1. Field of the Invention
The invention relates to a spiral distributor for a die head of a blown film extrusion line, a die head for such a blown film line, a blown film line, a method for manufacturing a blown film as well as a blown film manufactured by means of this method.
2. The Prior Art
The die head is the extrusion tool and thus the technological core of a blown film extrusion line. Regardless of its concrete configuration, the object of the die head is to mold the melt. The melt reaches the die head from one or several melt threads at the inflow of the tool and is to leave the die head with a uniform, thermally and mechanically homogeneous melt distribution via the outflow cross-section, which is shaped as an annular gap, downstream at the outflow of the tool.
The configurations of die heads customarily used today can be divided roughly into two groups: on the one hand, the group including spiral distributors with a cylindrical or conical shape, on the other hand, the group including radial spiral distributors, which are also called coil distributors.
The document DE 103 60 360 A1 shows a die head for a nine-layered film.
The book “Extrusionswerkzeuge für Kunststoffe und Kautschuk: Bauarten, Gestaltung und Berechnungsmöglichkeiten”; “Extrusion tools for plastic materials and rubber: configurations, design and calculation possibilities” by Walter Michaeli, with the cooperation of Ulrich Dombrowski . . . , second completely revised and extended edition; Munich, Vienna; Hanser-Verlag 1991, ISBN 3-446-15637-2” more specifically shows different spiral mandrel distribution tools starting at p. 159. On page 160 line 4f, the book describes how completely avoiding weld lines and flow marks is one of the main advantages of a melt distribution system.
The document DE 199 24 540 C1 discloses a cylindrical spiral distributor with a surrounding rotating mandrel.
Another cylindrical spiral distributor can be gathered from the document WO 88/01226 A1.
U.S. Pat. No. 6,866,498 B2 also shows a cylindrical spiral distributor in which outlets initially lead from a pre-distributor to deviating supply grooves. The supply grooves have ending areas. The spiral channels start at the end of the ending areas of the supply grooves.
It is common to all die heads, that the melt stream delivered by the extruder is initially divided into several individual flows. Star or ring-shaped distributor systems are predominantly used to this end. These so-called pre-distributors lead into the spiral-shaped channels, which are worked either into a mandrel (in the case of an axial, cylindrical or conical spiral distributor) or into a plate (in the case of a radial spiral distributor). The spiral channels run around the mandrel in the form of a multiple thread or are disposed in the form of a multiple spiral on the plate.
The depth of a channel usually decreases to zero in the direction of extrusion. The gap between the mandrel, respectively the plate, and an opposite side correspondingly increases. The gap between the mandrel, respectively the plate, and the opposite side thus becomes bigger. This causes a melt stream flowing in a spiral to be continuously divided into two parts: on the one hand a part which flows over the spillover bars which are located between two spirals; on the other hand a part which follows the course of the spiral channels.
Due to the multiple-screw-shaped or multiple-spiral-shaped overlap of the spiral channels, the melt issuing from a channel overlaps with the melt from other channels. The melt issuing from a channel reaches a gap flow. Due to the overlap of individual channel flows, so-called weld lines are avoided and a quite uniform thickness distribution of the melt strand at the end of the spiral distributor as well as a high thermal homogeneity of the melt is additionally achieved.
The depth of the channels in the spiral distributor usually decreases with a constant rise of the floor of the spiral channel.
There are also embodiments in which the depth decreases at such a rate, that the reduction of the cross-section of the spiral channel is constant along its course. This means that the channel floor becomes flatter at a continuously faster rate.